Method and device for determining the surface friction coefficient in bodies

ABSTRACT

The present invention relates to a process and device for determining the surface friction coefficients of bodies, in particular of catheters. In the invented process, the to-be-examined catheter ( 1 ) is pulled with a defined velocity through a gel-like viscoelastic substance ( 2 ) and the frictional force required therefor is measured. A defined pressure is applied to this viscoelastic substance in such a manner that a defined surface normal force acts on the catheter surface. By applying a given, defined pressure on the substance, the surface pressure on each area element of the catheter surface is precisely given and can be held constant. By holding the area to which pressure is applied constant, the entire acting normal force can also be held constant and the friction coefficient can be determined for any catheter diameter and for any catheter consistency. Determination of the friction coefficient is reproducible and only depends on the surface friction.

FIELD OF INVENTION

The present invention relates to a process and a device for determiningthe surface friction coefficients of bodies, in particular of catheters.

BACKGROUND OF THE INVENTION

Medical catheters are subject to various surface treatments to, i.a.,reduce the surface friction of the catheter in relation to thesurrounding tissue in order to permit easier and painless insertion ofthe catheter, for instance into bloodstream in the case of heartcatheters or into the urethra in the case of urinary catheters and toprevent possible injury to the tissue.

Catheters are often made of various, soft polymer materials, such as forexample silicon rubber or polyurethane, which partially undergodifferent surface treatment. The purpose of this surface treatment is tosmoothen and to hydrophilize in order to minimize surface friction aswell as to reduce adhesion of proteins (bacteria). Stronghydrophilization of the surface decreases reciprocal energy in aqueoussolutions and in that way the adhesion factor of frictional force.

Coats of different hydrogels, which feel slippery after submersion inwater, have proven to be especially low-frictional. However, hithertothere is no standardized method of quantifying this “slipperiness” inthe form of a surface friction coefficient so that presently it is notpossible to quantitatively evaluate a reduction in friction due to aspecific method of treating the surface.

Various attempts at measuring surface friction coefficients are alreadyknown in the state of the art. However, the measurement resultsdetermined with these known methods are not only dependent on thesurface friction but also on the configuration and the consistency ofthe respective measured catheter and, therefore, cannot be compared witheach other.

For example, many in vivo field tests were conducted for urinarycatheters to determine the surface friction in the urethra of rabbits aswell as humans (see Nickel, J. C. et al., “In Vivo Coefficient ofKinetic Friction: Study of Urinary Catheter Biocompatability”, UrologyXXIX (5), pp. 501-503 (1987); Khoury, A. E. et al., “Determination ofthe Coefficient of Kinetic Friction of Urinary Catheter Materials”, TheJournal of Urology 145, pp. 610-612 (1991); Tomita, N. et al.,“Biomaterials Lubricated for Minimum Frictional Resistance”, Journal ofApplied Biomaterials 5, pp. 175-181 (1994)).

These tests are very realistic, but are unsuitable as a standard testmethod for catheter coatings, because particularly the muscle tone ofthe urethra of the individuals participating in biological tests differ.Consequently, no defined surface pressure can be applied to the cathetersurface. As a result, the measured friction coefficients of a certaincatheter on various individuals differ greatly.

Various attempts are also known of laboratory test systems. Two reportsdescribe a test system in which two severed sections of the to-be-testedcatheter are attached to a glass plate and are then loaded with a planeblock of a defined weight coated with either collagen gel or hydrogel. Afriction coefficient is determined by measuring the force needed to pullthe block over the catheter (see Graiver, D. et al., “Surface Morphologyand Friction Coefficient of Various Types of Foley Catheters”,Biomaterials 14(6), pp. 465-469 (1993)) or by measuring the minimaloblique angle of the glass plate that is required to generate a slidemovement of the block (see Nagaoka, S. et al., “Low-friction HydrophilicSurface for Medical Devices”, Biomaterials 11, pp. 419-424 (1990)).Contrary to real systems, counterform active area pairing occurs inthese test systems, i.e. no full surface contact between the catheterand the block. Furthermore, surface pressure cannot be defined, becausethe contact surface varies depending on the softness and configurationof the tested catheter despite an unchanged weight load. Therefore, anormed friction coefficient cannot be determined with this method.

In two other methods (cf. Uyama, Y. et al., “Low-frictional CatheterMaterials by Photo-Induced Graft Polymerization”, Biomaterials 12, pp.71-75 (1991)), the catheter is pulled through a bent PVC tube and theforce required therefor is measured or a silicon rubber disc loaded at adefined weight is pulled over the catheter.

In both cases, there is largely conform active surface pairing, i.e. theareas of the catheter and the PVC tube respectively the silicon rubberdisc are in surface contact. In the first instance, however, the normalforce on the catheter cannot be defined and the size of the contactsurface also varies, because the catheter deforms when pulled throughthe rigid tube. In the second case, the normal force varies over thecontact surface and the contact surface itself also. Thus, the surfacepressure cannot be precisely determined for different sizes ofcatheters. In both methods, an unsuited material is selected in additionfor the counter body, because neither PVC nor silicon rubber ishydrophilic. Therefore, there is no similarity to real conditions whenusing catheters in the human or animal body.

Another attempt at determining the surface friction coefficients isknown from MARMIERI, G. et al., “Evaluation of Slipperiness of CatheterSurfaces”, Journal of Biomedical Materials Research (AppliedBiomaterials) 33, pp. 29-33 (1996). In the test method presented there,the catheter is pulled through a block of hardened agar. The timerequired to pull a piece of catheter out of the agar by means of adefined load of weight is set as the measure of the slipperiness of thecatheter.

However, no defined surface pressure on the catheter can be given withthis test system. Therefore, its does not represent a standardized testmethod.

Thus, none of the processes or devices known from the state of the artpermit standardized determination of the surface friction coefficient ofcatheters. The known processes yield measuring results that are not onlydependent on the surface friction but also on the configuration and theconsistency of the respectively measured catheter.

OBJECTS AND SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide astandardized process and a device for determining the surface frictioncoefficients of bodies, in particular of catheters, whose measuringresults are reproducible and only dependent on the surface friction andare independent of the configuration and the consistency of the materialof the body itself.

This object is achieved with a process and a device according to claims1 and 9. Advantageous embodiments are the subject matter of thesubclaims.

In the invented process, the to-be-examined body, for example acatheter, is pulled with a defined velocity through a gel-like, soft,viscoelastic substance and the friction force required therefor ismeasured. In this process, a defined pressure is applied to thisviscoelastic substance, the defined pressure spreading in the entiresubstance due to the viscoelastic material properties. As the catheteris enclosed in the substance for a specific length l( reciprocal actionlength), this pressure acts particularly also on the surface of thecatheter. Thus a defined surface normal force is applied to the cathetersurface. In this case, the entire charged area of the catheter isA=l*π*d, with d standing for the diameter of the catheter.

When measuring catheters of different diameters d, the invented processpermits holding the entire area A constant by adapting the reciprocalaction length l, on which the catheter comes into contact with theviscoelastic substance, conversely proportional to the diameter of thecatheter: l=A/(π*d). This can be achieved by adapting the size of theviscoelastic substance and the size of the container to receive thissubstance especially to each catheter diameter.

Measuring the force required to pull the catheter through theviscoelastic substance permits, with a given, known normal force,determining the surface friction coefficient of the catheter.

Of course, alternatively the viscoelastic substance can also be movedrelative to a stationary catheter at a defined velocity.

The surface pressure on every surface element of the catheter surfacecan be exactly defined and kept constant by applying a presettable,defined pressure to the substance. Holding the surface to which pressureis applied constant permits keeping the entire acting normal forceconstant for every catheter diameter and for every catheter consistency.Thus, for every catheter configuration and consistency, a defined normalforce exerted on a defined area of the catheter surface and the frictionforce can be measured. The ratio between the measured friction force andthe preset normal force yields the friction coefficient. Determinationof this friction coefficient is reproducible and depends only on thesurface friction. Therefore, the invented process and the devicetherefor permit quantitative evaluation of the reduction in friction byusing various methods of treating the surface of catheters.

The application of a preset, defined pressure to the substance ispreferably realized by applying a defined surface pressure to a freesurface of the substance. Thus, for instance, a surface pressure ofapprox. 10⁴ Pa can be applied to the free surface with the aid of awater column or by means of pumping in air or water into the containerwith the substance.

For a useful method of determining the surface friction coefficient ofcatheters, the given measurement conditions should be selected as closeto real conditions as possible. Therefore, it seems useful to select aphysiological saline solution with a temperature of 37° C. as theambient medium for measuring.

The viscoelastic substance should like biological tissue be ashydrophilic as possible and be reproducible in its mechanical andchemical properties and be producible temporally stable. Hydrophilic,synthetic gel-like materials, so-called hydrogels, are therefore moresuited as viscoelastic materials for this purpose than gels of naturalorigin.

In the following the present invention is made more apparent usingpreferred embodiments with reference to the drawings. They show in:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a diagram of the measuring principle of the present invention;

FIG. 2 a diagram of a preferred embodiment of a device of the presentinvention;

FIG. 3 a diagram of another preferred embodiment of a device of thepresent invention; and

FIG. 4a an example of a container according to the invented device;

FIG. 4b an example of an arrangement for filling the container accordingto FIG. 4a with a viscoelastic substance.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 shows again the invented principle of the present process using apreferred embodiment. A to-be-examined catheter (1) is pulled with adefined velocity v through a gel-like soft, viscoelastic substance (2),and the friction force F required therefor is measured. The substance(2) fills the interior of a rigid container (3) in such a manner that itsnuggles against the walls and floor of the container. This substancehas a cylindrical lumen running through its center. The diameter of thislumen largely corresponds to the diameter of the catheter. The walls ofthe container, too, are provided with boreholes (4), the diameter ofwhich is a little larger than that of the catheter, at both ends at thepoints of entry of the catheter so that the catheter does not rubagainst the wall of the container when pulled through the substance. Adefined surface pressure P is applied to the free surface (5) of theviscoelastic substance spreading, after a period of relaxation, in theentire substance due to its viscoelastic properties. This pressure actsparticularly on the surface of the catheter because the wall of thelumen running through the substance snuggles closely to the catheterwall due to its viscoelastic properties. Therefore, a surface normalforce defined by the surface pressure P is applied to the cathetersurface. The entire measurement can be conducted in a tub with anambient medium (6) comprising a physiological saline solution at 37° C.comparable to the real conditions in human body. The surface pressurefor urethra catheters range, for example, between approximately 4 cm H₂Oand 80 cm H₂O. A constant velocity v of 10 cm per minute, for example,can be selected for the relative movement between the catheter and theviscoelastic substance. However, the given values are, of course, onlyexamples: depending on the demands, other velocities and a differentsurface pressure, can of course, be used.

For measuring catheters, preferably hydrogels are employed as thematerial for the viscoelastic substance. Hydrogels containing phosphorylcholine, a molecule that is also present in biomembranes, areadvantageous. Furthermore, a hydrogel material (Hampshire's Hypol PreMAG 60) has advantages, because it is easy to work with. This is apolyurethane hydrogel prepolymer that cross links to form a polymerafter a few seconds upon mixing with water yielding a firm, hydrophilicsponge rubber. A clear, hydrophilic gel can also be produced by stirringthe prepolymer with water mixed with a large amount of acetone. In thiscase, the cross linking time increases to about half an hour permittingreadily pouring this material into molds during this time. After crosslinkage, the material has a firm, gel-like consistency with anabrasion-resistant, good wetting surface. After submersion in distilledwater, it easily comes loose from glass surfaces in such a manner that asmooth surface can be poured with it. In particular, a mixture of onepart prepolymer to eight parts acetone and two parts distilled waterproved to yield a gel consistency quite similar to that of firm muscletissue.

FIG. 2 shows an arrangement according to one of the preferredembodiments of the present invention. Herein the relative movementbetween a catheter (1) and the friction body in the form of a hydrogel(2) is achieved by the friction body being in a stationary definedposition and the catheter (1) being pulled through the gel lumen withthe aid of a mobile slide (9) at a defined velocity v. The tensileforce, which after subtraction of the offset of the catheter clamp (10)corresponds to the surface friction, required therefor is measured withthe aid of a force sensor (8) attached at the end of the catheter. Inanother preferred embodiment, shown in FIG. 3, catheter (1) is clampedtightly in a stationary frame (11) in such a manner that its positionremains fixed. The container (3) with the hydrogel (2) is then pulledwith the aid of a moveable slide (9) over the catheter (1) at a definedvelocity v. In this case, too, the force measured with the force sensor(8) attached at one end of the catheter corresponds to the surfacefriction if the clamping force held constant via the guide pulley (12)by means of the weight (13) is subtracted.

In both embodiments shown in FIGS. 2 and 3, the surface pressure can begiven in a defined manner, for example, by the hydrostatic pressure of awater column (here: physiological saline solution). A glass burettesuspended at a corresponding height (well 1 m over the catheter) from ameter rule, for instance, can serve as a container for the water column.The bottom end of the burette is connected to the hydrogel container viaa flexible hose connection.

A piece of tube positioned horizontally in axial direction and closed atboth ends by a sealing lid (14) with a central opening (4) respectivelyfor passage of the catheter can serve as the container (3) for thehydrogel (as viscoelastic substance). An example of such a container isshown in FIG. 4a. The container length suited for the respectivecatheter size can then be obtained simply by inserting a tube shortenedto the required length and filled with hydrogel. One of the lids isprovided in addition with an opening (15) through which pressure can beapplied to the interior of the container.

As preparation for measurement, the tube is transformed into a mold byclosing suited lids (16) at both ends as shown in FIG. 4b. In order toobtain a required lumen in the hydrogel mass, a stainless steel or glassrod (17) is enclosed inside the hydrogel (not shown). The diameter ofthe stainless steel or glass rod (17) corresponds largely to thediameter of the to-be-examined catheter. The rod is removed followingcomplete vulcanization of the hydrogel and a correspondingly dimensionedrecess remains in the gel.

A length of the hydrogel container (3) of, for example, 150 mm for acatheter diameter of 2 mm yields a contact surface of 942 mm² due towhich, with a hydrostatic pressure of, for instance 10⁴ Pa (approx. 1 mwater column), the overall normal force of 9.42; N acts on the cathetersurface. Assuming a catheter surface friction coefficient in thevicinity of μ=0.1, one measures with the described measuring device afriction force of about 1N. For larger catheter diameters, one needscorrespondingly shorter hydrogel containers in order to keep the contactsurface constant. For example, a container with a length of 60 mm isused for a catheter with a diameter of 5 mm.

With the invented process and the device therefor, the same surfacepressure can be applied to catheters of different diameters in such amanner that a direct comparison of the determined surface frictioncoefficient is possible independent of the configuration and consistencyof the respective measured catheter.

The invented process and device therefor are, of course, also suited fordetermining the surface friction of other bodies as catheters. Thepreferred application relates to bodies that come into contact withbiological tissue, i.e., are inserted into, for example, thebloodstream, urethra or the like.

What is claimed is:
 1. A process for determining a surface frictioncoefficient of a body, comprising enclosing the body in a gel-likeviscoelastic substance over a reciprocal action length, generatingbetween said body and said substance a relative movement with constantvelocity and measuring a force required therefor, wherein a presettable,defined pressure is applied to said substance at least during saidrelative movement; said reciprocal action length is selected in such amanner that a defined surface normal force acts via said substance on anarea of said body preselectable by said reciprocal action length; andsaid surface friction coefficient is determined from the ratio of themeasured force to the surface normal force.
 2. The process according toclaim 1, wherein said substance has a free surface to which a defined,given surface pressure is applied to generate said defined pressure. 3.The process according to claim 2, wherein pressure is applied to saidfree surface of said substance via gas or water pressure.
 4. The processaccording to claim 1 or 2, wherein said body is a catheter.
 5. Theprocess according to claim 4, wherein with catheters of differentdiameters d_(i), said reciprocal action length l_(i) is selected foreach said catheter in such a manner that on each said catheter pressureis applied to the same constant area A=l_(i)*n*d_(i).
 6. The processaccording to claim 1, wherein said body is pulled through saidsubstance.
 7. The process according to claim 1, wherein said substanceis pulled over said body.
 8. The process according to claim 1, whereinsaid substance is a hydrogel.
 9. A device for determining a surfacefriction coefficient of a body, based on a ratio of measured force to asurface normal force, comprising a container containing a gel-likeviscoelastic substance in which the body is enclosed over a reciprocalaction length, a relative movement with constant velocity beinggenerated between said body and said substance and a force requiredtherefore measured, said container having an inlet opening and an outletopening which permit pulling said body through said substance in orderto generate the relative movement between said body and said substanceand another opening for applying a presettable defined pressure to saidsubstance at least during the relative movement.
 10. The deviceaccording to claim 9, further comprising a slide for attaching said bodywith which said body can be moved relative to said container withconstant velocity.
 11. The device according to claim 9, furthercomprising a slide for attaching said container with which saidcontainer can be moved relative to said body with constant velocity.